Vector network analyzers (VNA) have in the past been used to measure the continuous wave (CW) S-parameter performance of devices being tested. Typically the CW signal will be a radio frequency (RF) signal in the range of 500 MHz to 50 GHz, but this range can vary. Often under these normal operating conditions the VNA is functioning as a narrowband measurement instrument, where the VNA applies a known RF signal to the device under test and measures the RF frequency response. If one were to look at the response of a single CW frequency one would see a single spectral tone in the frequency domain. Typically a VNA analyzer has a built in source and receiver which are designed to operate together in a synchronous manner, utilizing narrowband detection, to measure the frequency response of the device under test (DUT). Most analyzers can be configured to generate a frequency sweep over many different frequency ranges.
In some cases the signal applied to the DUT must be pulsed (turned on and off) at a specific rate (sometimes referred to as the pulsed repetition frequency (prf)) and duration. If one were to look at the frequency domain response of a single pulsed tone, it would contain an infinite number of spectral tones making it challenging to utilize a standard narrowband analyzer.
For example, a single pulsed tone that results from toggling a sine wave x(t) on and off is illustrated as 100 in FIG. 1A. This single pulsed tone 100 is expressed as rectpw(t)·x(t). The single pulsed tone 100 may be convolved with the following series function:
  shah  ⁢      1    prf    ⁢      (    t    )  illustrated as 102 in FIG. 1A to obtain a pulsed signal y(t), illustrated as 104 in FIG. 1A. This convolution is expressed by the equation:
      y    ⁡          (      t      )        =            (                                    rect            pw                    ⁡                      (            t            )                          *                  x          ⁡                      (            t            )                              )        *    shah    ⁢          1      prf        ⁢                  (        t        )            .      
FIGS. 1B and 1C provide further illustration of the spectral components of a pulsed RF signal in the frequency domain. Specifically, FIG. 1B shows a graph 105 of a signal 106 which shows a broad spectrum of the spectral components in the pulsed RF signal. The pulsed RF signal in the frequency domain 106 shows minimum points 109 in the spectral components at a spacing of the 1/pulse width). FIG. 1C shows a much higher resolution graph 107 of the spectral components surrounding the primary spectral component 108, where the primary spectral component is at the frequency of the RF signal, which is generated by an oscillator and then the output of the oscillator is typically toggled on and off to provide a pulsed RF signal. Additional spectral components are then offset by the pulsed repetition frequency (prf) (the frequency at which the RF signal is turned on and off). In the case of FIG. 1C the prf is approximately 1.69 KHz. The different spectral components 110, 112, 114, and 116 are then offset from the primary spectral component by a frequency amount equal to an integer multiple of the prf.
In some applications pulsed signal device characterization can use adaptive filter nulling and filter gating to isolate the primary spectral component which corresponds to the RF frequency of the RF signal which is being toggled on and off to provide the pulsed RF signal. One example of such an application is provided in the pending U.S. patent application Ser. No. 10/883,100, entitled PULSED SIGNAL DEVICE CHARACTERIZATION EMPLOYING ADAPTIVE NULLING AND IF GATING which is assigned to the same assignee as the present application, and which is incorporated herein by reference in its entirety. It will be recognized that in general the primary spectral component can correspond to an intermediate frequency (IF) which results from a receiver channel of the VNA down converting the RF sinusoid of the pulsed signal after it has been incident upon the device under test.
A filtering technique which uses adaptive filter nulling and gating to exclude spectral components other than those corresponding with the primary spectral component or the corresponding IF can, however, result in a loss of energy from the original measurement signal which is received from the DUT. Generally this loss of energy is proportional to the duty cycle of the pulsed RF signal due to the filter rejecting everything except the fundamental tone (the frequency of the RF signal which is pulsed on and off) of the pulsed signal. As the duty cycle decreases, more energy moves into the sidebands and less energy remains in the primary spectral component. This is because the magnitude of the sideband spectral components in the frequency domain are inversely proportional to the product of the pulse width (PW) and the pulse repetition frequency (i.e. Duty Cycle=(PW×PRF)). Thus, where adaptive filter nulling and gating is used, the duty cycle can have a direct effect on the measurement dynamic range since the processed signal energy is reduced when the duty cycle is decreased. This loss in processed signal energy is especially problematic for signals pulsed at very low duty cycles such as required in isothermal measurements and device characterization techniques. In some cases such measurement results are unusable because of the lack of system dynamic range.